Laboratory study of flashover performance of polluted and ice-covered insulators exposed to impulse overvoltage

Summary

Line insulators are subjected to environmental, electrical and mechanical stresses. The presence of an ice layer of pollution or ice on an insulator can significantly reduce its flashover performance. The probability of insulation breakdown rapidly increases when the insulator is exposed to a combination of pollution, ice and transient overvoltage. The understanding of arc initiation and propagation under such conditions is very important for insulator design and insulation coordination. The flashover performance of an insulator covered with a polluted ice layer was investigated by generating three impulse voltages of different shapes: lightning impulse with front time of 1.2 µs and half time 50 µs, switching impulse with front time 100 µs and half time 2500 µs and switching impulse with front time 250 µs and half time 2500 µs. These impulses are representative of different transient situations in electrical power systems. The ice on the surface of the insulator was created inside a climate room from supercooled droplets produced by a spraying system through a uniform airflow from a system of fans, at CIGELE laboratories. The insulator was energized by ac operating voltage during the ice accumulation phase. Water conductivity was adjusted by adding sodium chloride (NaCl) to de-ionized water. Measurements were performed for three different ice thicknesses in a range up to 3 cm to reflect climatic conditions prevalent in Central and Eastern Europe. The ice quantity was determined by measuring the ice thickness on monitoring cylinder which was placed in the place of tested insulator. The flashover voltage and volt-time characteristics for lightning and switching impulses of both polarities were determined. Laboratory experiments show that an increase in front duration of impulses and ice thickness have a tendency to decrease the flashover voltage. The flashover voltage reduction of the tested suspension string of cap and pin glass insulators reached the value of 25 % for positive polarity and 44 % for negative polarity.

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Authors

Prochazka Radek, Bartunková Monika, Tlusty Josef, Chvojka Jirí